Bone cement is typically prepared from a polymer powder component and a liquid monomer component by thorough mixing to form a paste-like flowable mass which then hardens and/or binds after being introduced, for example, into the medullary channel of the femur of a patient for the purpose of anchoring the shaft of a hip replacement prosthesis, thereby ensuring a permanent load-bearing attachment of the prosthesis in the femur. It is known since many years that the mechanical stability of bone cements is substantially reduced by greater and smaller air inclusions which are introduced into the resulting cement matrix in particular when the cement components are mixed. The air bubbles enclosed in the cement produce pores which can lead to the formation of fissures and gaps in the cement when subsequently stressed by the prosthesis. This can prematurely destroy the cement jacket surrounding the prosthesis which in turn can loosen the prosthesis, necessitating its removal. Experimental and clinical studies have shown that substantially air-free and hence also pore-free cements contribute to an increased fatigue resistance and thereby extend the lifetime of the endoprosthesis. Mixing systems for bone cement have been developed, whereby the mixing process is carried out under vacuum. For example, mixing vessels have been developed, whereby the vessel is closed off after the components are introduced, so that monomer vapors cannot escape during the mixing process. In other systems, the mixing process is carried out under reduced atmospheric pressure after the mixing vessel is filled. In this case, the monomer vapors are typically suctioned off with a vacuum pump and trapped in a charcoal filter. Air inclusions during mixing can be significantly reduced by reducing the pressure, which significantly increases the stability of the bone cement and therefore also the lifetime of an endoprosthesis anchored with bone cement in the bone.
It is therefore an object of the invention to provide a device for preparing, i.e. thoroughly mixing, the components of an exemplary bone cement under vacuum, wherein the device is comparatively simpler than conventional systems and enables application of completely mixed bone cement in the designated attachment region while the mixed bone cement is still in a flowable paste-like state. Vacuum is applied only during the mixing process of the bone cement component, whereas the actual application is carried out with the device after the device has been vented, i.e., under atmospheric pressure.
Based on a device of the aforedescribed type, the object is solved by the invention in that the sealing cap, which can be placed over the edge of the open end of the cylinder, is movably connected to at least one elongated locking element which can be inserted through corresponding recesses provided in the edge region of the cylinder and through corresponding openings provided in the piston. When the locking element(s) is/are inserted, it/they hold(s) the sealing cap and the cylinder on or in the open end region of the cylinder. Locking the piston with the sealing cap ensures that the piston, which is mounted on the cylinder together with the sealing cap after the material components to be mixed have been filled in, is secured in the upper end position due to the fact that the cap and the piston interlock when the mixing mechanism is operated by simultaneously displacing and rotating the mixing shaft. Mixing takes place in vacuum, as required, which is introduced by the vacuum source via the vacuum fitting.
According to a preferred embodiment of the invention, the locking element(s) is/are formed as elongated flat locking bracket(s), with the sealing cap pivotally joined with the locking bracket(s). When the sealing cap and the locking bracket(s) are formed as injection molded plastic parts, the locking bracket(s) can be hingedly connected to the sealing cap via an integrally formed film hinge. According to a preferred embodiment of the invention, two mutually parallel locking brackets are provided that have flat side surfaces located in a plane.
According to another advantageous embodiment of the invention, the locking brackets are pre-mounted in a corresponding through-opening of the piston, which through-opening penetrates the piston perpendicular to the piston""s longitudinal center axis and is complementary to the cross-section of the locking brackets. Open recesses which have a narrowed mouth adapted to receive the locking brackets are provided in the end region of the open cylinder. When the piston is in the intended locked position, the locking brackets can be elastically pressed together in the transverse direction in a region located at the open end of the cylinder. Accordingly, the piston held by the locking brackets together with the sealing cap can be secured in the open end by elastic compression in the transverse direction.
Advantageously, the locking brackets can be elastically compressed in the transverse direction in the region of the recesses near the edge of the cylinder as a result of elongated through-holes disposed in the locking brackets in a region that in the intended locking position is located in the recesses of the cylinder, wherein the through-holes permit a relative deformation of the remaining edge regions.
The piston, the mixing shaft which movably penetrates the piston and is provided on its, cylinder-interior end with the mixing mechanism and on the exterior end with a handle, as well as the sealing cap that is penetrated by the mixing shaft, together with the latching brackets that are pre-mounted in the through-openings in the piston, are pre-mounted in the form of an assembly that can be inserted in the open end of the cylinder and locked by engaging the locking brackets in the recesses disposed in the edge region of the open end of the cylinder. The pre-mounted assembly can be inserted into the cylinder as a unitxe2x80x94after the cylinder has been filled with the bone cement componentsxe2x80x94, whereby the piston and the sealing cap interlock in the open edge region. After interlocking, the connecting line to the vacuum source can be connected to the vacuum connection on the piston. When a vacuum pressure suitable for mixing is attained in the cylinder, the components are mixed by moving the mixing shaft in and out and simultaneously rotating the mixing shaft, whereby the mixing mechanism processes the components to be mixed into homogeneous flowable bone cement.
After mixing is complete, the mixing shaft is retracted until the mixing mechanism contacts the end face of the piston inside the cylinder, whereafter the mixing shaft is broken off in a region of the mixing shaft positioned outside the cap.
Advantageously, the mixing shaft, when the mixing mechanism is moved completely into a position so as to contact the cylinder-internal end face of the piston, is provided in the region of the outer end face of the piston with a rated break point, where the portion of the mixing shaft positioned outside the piston can be broken off. Due to the tiltable connection of the sealing cap with the locking brackets, the sealing cap can be tilted upwardly, whereby the connection line to the vacuum source is simultaneously pulled off the corresponding suction connection in the piston, venting the interior of the cylinder. After the sealing cap is tilted upwardly, the locking brackets can be pulled out of the recesses in the edge region of the cylinder and through the openings in the piston. The cylinder can then be pushed downwardly, by hand or by using the broken-off mixing shaft, towards the opposing end which is still closed off by a screwed-in sealing plug, thereby compacting the bone cement to a dense mass. The cylinder is then inserted in this condition into a customary application gun, with the piston of this application gun pressed into the end face of the piston on the outside of the cylinder, until contract is established. The sealing plug, which hitherto prevented bone cement from leaking out, is then replaced by an elongated application nozzle, also referred to as xe2x80x9csnorkel,xe2x80x9d and the plastic bone cement mass can be pressed out through the snorkel by operating the application gun. If the application nozzle is used to set up the shaft of an endoprosthesis in the medullary channel of a bone, it should preferably be of sufficient length so as to extend up to the medullary space plug inserted into the medullary space before the shaft of the endoprosthesis is inserted. The bone cement is then filled by operating the application gun to fill the space from the medullary space plug to the open end of the bone, whereby tissue water, blood and other contamination are pushed out by the rising level of the bone cement, leaving only bone cement in these provided space after the shaft of the endoprosthesis has been inserted into the medullary channel filled with bone cement.
The rated break point, which allows the shaft to be easily broken off, is advantageously not disposed on the outside of the mixing shaft, because the required notches could otherwise damage a sealing ring which seals on the mixing shaft.
According to an advantageous embodiment of the invention, the mixing shaft is formed by a small tube having a through-bore extending along its entire length, wherein the rated break point is formed by a circumferentially notched indentation disposed in the interior wall of the small tube.
Advantageously, the vacuum fitting in the piston is formed by a vacuum nipple having a vacuum channel which is guided through the piston into a ring-shaped circumferential recess disposed in the cylinder-side end face of the piston, wherein the ring-shaped circumferential recess is closed off to the inside of the cylinder by a circular porous disk made of a filter material having an average pore size selected to be permeable to gas, but impervious to particles of the powder or granulate component filled in the cylinder and/or to the prepared paste-like mass.
Advantageously, the circular disk made of filter material is held on the cylindrical end face of the piston by a clip ring having a larger diameter and extending over its outer edge and/or by a clip ring having a small diameter and extending over its inner edge.
According to another advantageous embodiment, the clip ring having the larger diameter can include an attachment section that grips over a circumferential section of the piston and has a diameter that is smaller than the unobstructed inside diameter of the cylinder, and/or the clip ring having the smaller diameter can include an attachment section that engages with a bore section having a diameter that is larger than the diameter of the through-opening provided in the piston for the mixing shaft.
A corresponding sealing ring can be disposed in the groove-shaped recesses formed between the clip ring attachment sections and the ring-shaped surfaces produced in the piston in a region where the diameter of the circumferential section or bore section changes, by dimensioning the attachment section of the clip ring that grips over the circumferential section of the piston with the smaller diameter and/or the attachment section of the clip ring that engages with the bore section in the piston with the larger diameter to be shorter in the direction of the longitudinal center axis of the piston than the corresponding circumferential or bore section of the piston. The sealing ringsxe2x80x94which can be formed, for example, by O-ringsxe2x80x94can be easily mounted, before the clip rings are placed, by pushing or inserting them in the attachment sections disposed in the piston.
Advantageously, the clip rings holding the disk made of filter material on the piston can be attached by interlocking the attachment section of the respective clip ring, which grips over the circumferential section of the piston and/or engages with the corresponding bore section, on or in the associated section of the piston, and/or holding the attachment section with an adhesive and/or by a press fit.